Responsive sites adjust fluidly to different screen and viewport sizes - from the phone, to the tablet, to your 27 inch iMac. It's both an art and a science. There are hundreds of examples of well-done responsive sites on the internet, but these are currently my favorite five.
Note: I adjust this list as I come across new sites that are able to break into the top five, so it will change with time.
Not only is the United Pixelworker's site a wonderful implementation of a responsive site, but it's done in a notoriously difficult genre to do responsively - E-commerce. I love how the product photos flow as the screen size changes. I also like the thought they've put into the site header for each screen size. Well done.
The changing animation header is worth a look just by itself, but there are other little details that make this site appealing. Notice how color themes change with size, background images rotate, and the menu becomes an icon.
Microsoft's new homepage, as created by Paravel is clean, simple, and very attractive. I enjoy how they've rethought the photo slider captions at different screen sizes.
The responsive design is just one thing to love about Jessica Hische's site. Known more for her lettering than her web design, you couldn't tell it by looking at this site, it's beautiful. The whole site just feels right at every screen size.
5) Simple Bits
Dan Cederholm always produces wonderful work, and his home site is no exception. His site is a clean and minimalist design, and it flows well at every screen size. I enjoy the small touch of the menu link subtitles dropping off as the viewport shrinks.